1. Field of the Invention
This invention relates to a device for suppressing voltage fluctuation and higher harmonics of the power system that are generated due to power fluctuation of the load with a large amount of power fluctuation and higher harmonic current.
2. Description of the Related Art
Conventionally, when suppressing voltage fluctuations of a power system generated from a load with irregular fluctuations of reactive power and large capacity, such as an arc furnace, it has been the practice to control the reactive power of a voltage fluctuation suppression device, which was provided in parallel with the load in order to compensate for these fluctuations of reactive power, by detecting the reactive power consumed by the load.
FIG. 4 shows a circuit layout of a conventional voltage fluctuation suppression device, which is disclosed, for example, in U.S. Pat. No. 4,752,726 (June 21, 1988). In FIG. 4, to avoid complicating the drawing, the three-phase system is shown as a single line connection diagram. In FIG. 4, power is fed to a load 1 from a power source 2 through a transmission line 3 (asssumed to be of impedance jXs) and a receiving transformer 4 (assumed to be of impedance jXt). A voltage fluctuation suppression device constituted by a self-commutated converter 10 is provided in parallel with the load 1. This includes a control circuit 20 wherein the reactive power is calculated from the current and voltage detected by a current transformer 5 and a voltage detection transformer 6, and a current to compensate for the reactive power is supplied. As shown in FIG. 5, the self-commutated converter 10 includes ac reactors 12, gate turn-off thyristors (GTO) 13, diodes 14, and a dc capacitor 15. The current of the load 1 detected by the current transformer 5 is the line current of the three-phase circuit and is converted to the two-phase ac by a three-phase/two-phase conversion circuit 21. This process can be represented by the following equation, where i.sub.R, i.sub.S and i.sub.T are the respective line currents of the three-phase ac, and i.sub.d and i.sub.q are the currents of the two-phase ac. ##EQU1##
In the same way, the circuit voltages v.sub.R, v.sub.S, v.sub.T are detected by the voltage detection transformer 6, and converted to the two-phase ac by a three-phase/two-phase conversion circuit 22. The expression representing this conversion is the same as equation (1), substituting v for i. ##EQU2##
The voltage signals v.sub.d and v.sub.q obtained by this two-phase conversion are converted by a synchronization detection circuit 24 into synchronized voltage signals v.sub.d *, v.sub.q * synchronized with the fundamental component. A reactive power detection circuit 23 detects the so-called instantaneous real power and instantaneous imaginary power, as defined in Article No. 58-B60, P. 41 to 48 of Denki Gakkai-Shi Ronbun (The Journal of the Electrical Association of Japan) "Generalized Theory of Instantaneous Reactive Power and its Application". These are found by calculation by the following expression: ##EQU3## where p is the instantaneous real power and q is the instantaneous imaginary power. In the two-phase ac, v.sub.d * and v.sub.q * are orthogonal components of magnitude 1 pu, and v approximately equals v.sub.d * and vq approximately equals v.sub.q *. p is therefore the instantaneous active power supplied from the power source 2 to the load 1, and q is the instantaneous reactive power circulating between the two phases.
Also in the power system, the magnitude of the voltage fluctuations due to reactive power is dominant, and the fluctuations due to active power can be neglected. The self-commutated converter 10 need therefore only compensate q. The compensation amounts q* can therefore be obtained by an inverter circuit 25 by inverting q* the sign of q, as EQU q*=-q (4)
An instantaneous current calculation circuit 26 calculates the respective line current command values i.sub.CR *, i.sub.CS * and i.sub.CT * to make the self-commutated converter 10 act as a current source. These are found by inverse conversion of expression (3) and expression (1). Taking the command values in the two-phase ac as i.sub.cd * and i.sub.cq *, these are given by: ##EQU4##
Since, for the reasons discussed above, p*=0, we have: ##EQU5##
An calculation circuit 27 calculates the differences between the respective currents i.sub.CR, i.sub.CS and i.sub.CT of the self-commutated converter 10 detected by the current transformer 11 and the command values found from the above equation. An error amplifier 28 uses these differences to perform constant-current control, tracking the command values of the converter 10. The outputs of the error amplifier 28 are input to an gate circuit 29, generating on/off pulses for PWM control of the converter 10, these pulses being applied to the gates of GTOs 13 shown in FIG. 5. Since this self-commutated converter 10 is of the voltage type, the dc capacitor 15 is needed to make the dc voltage constant. An ac reactor 12 performs the action of smoothing the pulse-width modulated voltage and converting it to a current.
While voltage fluctuations are controlled as above, a higher harmonic filter 7 consisting of a reactor and a capacitor is provided in order to suppress higher harmonics generated by the load 1. Such a filter is a so-called single harmonic tuning type passive filter, so a group of higher harmonic filters are constituted by providing one or more higher harmonic filters for the higher harmonics of each order.
Thus, in a conventional device for suppressing voltage fluctuation and higher harmonics, higher harmonic filters were used as higher harmonic suppression means. However, such a higher harmonic filter cannot suppress all higher harmonics, it cannot suppress higher harmonics corresponding to the antiresonance frequency determined by the constant of the higher harmonic filter and the reactance Xs of transmission line. In particular in the case of an arc furnace, when the higher harmonics vary steeply and irregularly, the higher harmonics cannot be suppressed by the higher harmonic filter, and may in fact be increased.
The voltage fluctuation and higher harmonic suppression device as shown in FIG. 4 wherein the higher harmonic filter 7 and the self-commutated converter 10 are connected in parallel to the load 1 has in principle the capability of suppressing higher harmonics. But, since its control is performed using an open loop, if there is some control error, higher harmonics may be increased by the phenomenon described above, or at any rate sufficient benefit is not obtained.
Also, if one attempts to suppress all the higher harmonics generated by the load 1 by means of self-commutated converter 10, the higher harmonic filter 7 ends up simply having the function of a static capacitor, and the capacity of the self-commutated converter 10 itself becomes large, which is uneconomic.